check_mk: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/check_mk - from package: check-mk

You may install the required package with command yum install {package}

Error in other languages:
check_mk: bevel nie gevind nie
check_mk: comandă negăsită
check_mk: comando não encontrado
check_mk: commande introuvable
check_mk: command not found
check_mk: käsku ei ole
check_mk: không tìm thấy lệnh
check_mk: komanda nerasta
check_mk: Kommando nicht gefunden.
check_mk: kommandot finns inte
check_mk: komut yok
check_mk: nie znaleziono polecenia
check_mk: níor aimsíodh an t-ordú
check_mk: no se encontró la orden
check_mk: no s'ha trobat l'ordre
check_mk: opdracht niet gevonden
check_mk: parancs nem található
check_mk: perintah tidak ditemukan
check_mk: príkaz nenájdený
check_mk: příkaz nenalezen
check_mk: команда не найдена
check_mk: командата не е открита
check_mk: 命令找不到
check_mk: コマンドが見つかりません